PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 20091

Upper Limit on the Diffuse Gamma Ray Flux using GRAPES-3Experiment
M. Minamino∗ , S.R. Dugad†, T. Fujii∗ , U.D. Goswami†, S.K. Gupta† , Y. Hayashi∗, N. Ito∗ ,S. Kawakami∗ , H. Kojima‡ , T. Matsuyama∗, H. Miyauchi∗ , P.K. Mohanty† , S.D. Morris†,
P.K. Nayak† , T. Nonaka∗, S. Ogio∗ , T. Okuda∗ , A. Oshima† , M. Sasano∗,
K. Sivaprasad†, H. Tanaka∗, S.C. Tonwar†, E. Usui∗ and Y. Yamashita∗∗ Graduate School of Science, Osaka City University, Osaka 558-8585, Japan† Tata Institute of Fundamental Research, Mumbai 400005, India‡ Aichi Institute of Technology, Aichi 470-0392, Japan

Abstract. Isotropic gamma ray flux is expected
as a result of interactions of ultra high energy
cosmic rays (E > 1019 eV) with the cosmic microwave
background radiation and/or the annihilation/decayof topological defects. On the other hand, diffuse
gamma ray flux from the galactic plane is expected
as a result of interactions of cosmic rays with theinterstellar matter. Thus these gamma rays are oneof the possible probes to study cosmic rays and theirsource distribution. We search for diffuse gamma
rays with the GRAPES-3 air shower array. It is ahigh-density array of 1 m2 area plastic scintillation
detectors and the large area (560 m2 ) shielded muon
detectors, and it is located at Ooty in the southern
India. High statistics with such large area muon
detectors help us to distinguish gamma ray showers
with hadron showers. Based on the observations fromMarch 2000 to July 2004 we obtained a new upper
limit of the ratio of isotropic gamma rays to cosmic
rays in the energy range of 20 – 400 TeV, and also
we obtained an upper limit of galactic gamma rayflux (|b| < 2◦ ).
Keywords: diffuse radiation, gamma rays, airshower observation

Fig. 1. The GRAPES-3 air shower array; The detectors in the regions
bounded by the solid line and dotted line are the trigger detectors.

I. I NTRODUCTION5), the diffuse gamma ray flux is expected to have a
excess on the galactic plane.
Based on the observations with the GRAPES-3 air
shower array, we searched for diffuse gamma rays andhear we report upper limits of the isotropic flux and of
the galactic component.

There are a variety of possible sources of the diffuse
radiation in very high energies. We make a list of
possible astrophysical scenarios below;
1) unresolved point sources in extra galactic astronomical objects [1] [2]2) electromagnetic cascades induced by the interaction of ultra high energy cosmic rays (UHECRs)
with the cosmic microwave background radiation [3]3) radiation induced by the annihilation/decay of
topological defects [4]4) the inverse Compton scattering of cosmic rayelectrons with interstellar photons [5] [6]5) cascading products by the collision of cosmic rays
with interstellar medium [7]In scenarios 1) to 3), an isotropic diffuse gamma ray
flux is expected. On the other hand, in scenarios 4) andThe GRAPES-3 experiment is located at Ooty (N11.4, E 76.7 and 2200 m a.s.l.) in the southern India. It
consists of the two types of detectors, namely the surface
scintillation detectors and the shielded muon tracking
detectors under thick absorber material. It can detect airshowers induced by cosmic rays and gamma rays in theenergy range from 10 TeV to several tens of PeV with anadequate triggering condition. The 257 surface detectorsof 1 m2 area consisting of 5 cm thick plastic scintillator
are located on the hexagonal grid of 8 m span. In 17 Dec.
2003, more 32 detectors were deployed near the muon
detectors, then the total number of the surface detectors
is 289 (see Fig. 1).
Moreover, there are 16 muon detector modules. One
module is a stack of four layers of the propotional

II. GRAPES-3 E XPERIMENT2M. MINAMINO et al. GRAPES-3 DIFFUSE EMISSION

counter tubes (the cross-sectional area of 10 × 10 cm2
and the length of 6 m), and the layers are separated by
the 15 cm thick concrete plate each other. The numberof the propotional counters per layer is 58, thus thedetection area is 35 m2 per module. Each module iscovered with 2 m thick concrete absorber, and it isequivarent to the detection threshold energy for muonsof 1 GeV.
The triggering condition is any ten or more surface
detector hits out of 120 detectors in the central area ofthe array. The triggering rate was about 13 Hz, and after
the additional deployments it became about 24 Hz.
III. A NALYSIS
We analyzed data from March 2000 to July 2004 when
the total live time is 1389 days. The data recorded by thesurface array are used for the estimation of the primaryenergy and the core location of air showers. The dataobtained by the shielded muon detector are used for theselection of primary gamma ray candidates.
A. The Surface Array
In the analysis for the surface array data we used thefollowing event selection criteria:1) Air shower core should be inside a hexagonalbounded by the line (a) or (b) which are shownin Fig. 1.
2) Ratio of the detected particles by outermost tworings to the total detected particles < 35%.
3) Age parameter, s < 1.84) Number of hit ADC (NADC ) ≥ 75) Number of hit TDC (NTDC ) ≥ 156) 0.5 < NADC /NTDC < 2.0
The core location, arrival direction, and total numberof detected particles of the electromagnetic component
were determined for each shower. The core location isestimated to be the centroid of the seven largest hitdetectors. The arrival direction is determined from fitting
the hit timings to a conical air shower front structure.
In order to avoid misidentifying a large air shower
falling out of the array as a small shower we used thecriterion 2). The selection efficiency for the criterion
was calculated with semi-Monte Carlo simulation basedon GENAS ver 2.2. As a result the contamination ofshowers out of the array is estimated to be < 1% in theenergy range > 30 TeV.
B. Shielded Muon Detectors
In the muon component analysis we calculate thenumber of muons from the observed tracks in thedetector modules which have the same direction of airshowers. We identify a track with any three hit-layer
coincidence out of four layers. The number of detectedmuons depends on the primary energy, core distancefrom the muon detectors. Typically in our experiment
several muons are detected for a shower with the total
hits of 100 particles and with the distance from the muon
detectors of 40 m.

While the gate width is 10 µs, in order to reducechance muons we normally apply the narrower software
gate of 3.5 µs. The counting rate of muon tracks,
unrelated to air shower triggers, is about 3900 Hz permodule. Therefore, the average number of muons dueto the chance coincidence is very small, estimated to beabout 0.07 per event.
IV. S IMULATION AND C ALCULATION
Many theoretical and numerical researches predictthat the number of muons in gamma ray showers aremuch less than that in hadron showers at the same
primary energy. For example, a detailed Monte Carlosimulation for our experiment with CORSIKA ver 5.62
shows that the detected number of muons is expected
to be reduced to 2 – 3% of the number of muons byproton showers. Since the typical events observed in theGRAPES-3 air shower array have 0 to several tens ofmuons, we classified the obtained data into two groups:
one is muon-poor, that is, the group of air showers
with no associated muon, and the other is remainder,
and then we assumed muon-poor events are gamma ray
candidates.
We calculated the upper limit of the ratio of gamma
rays to cosmic rays. For this analysis, we calculated thegamma ray selection efficiency, ǫγ , that is the ratio of
the number of remaining gamma ray candidates after
the muon-poor selection to the incident primary gamma
rays with the Monte Carlo simulations. The efficiencies
are obtained for the different air shower sizes and corelocations.
V. R ESULTS AND D ISCUSSION
A. Isotropic Gamma Rays
We select the events with the zenith angle less than25◦ , and we calculate the upper limit of the ratio ofgamma rays to cosmic rays by:
µ=0

N
11Iγ
≤ 90%C.L.
ICRNall ǫγ 1 − nchanceµ=0where N90%C.L.
is a 90% confidence level upper limit on
the number of muon-poor air showers assuming Poisson
distribution, Nall is the total number of air showers.
nchance is the average number of muons due to a chancecoincidence [8] [9] [10]. The result is shown in Fig. 2and compared with the upper limits given by othergroups [11] [12] [13] [14].

B. Galactic Diffuse Gamma Rays
We select the events with the zenith angle less than30◦ , and we define the region of the galactic latitude|b| < 2◦ as the galactic plane region and 4◦ ≤ |b| < 20◦
as the background region. Then we compare the numberof events on two regions for different shower size asshown in Fig. 3.
As a result we cannot find any significant excess on
the galactic plane. The significance and the involved

PROCEEDINGS OF THE 31st ICRC, ŁÓDŹ 2009

3TABLE I
I NNER G ALACTIC PLANE
>E
(TeV)
17.6
27.9
44.3
70.2
111
176

Significance
(σ)
0.72
0.15
−0.57
2.23
−0.48
−1.51

90%C.L. upper limit(cm−2 s−1 sr−1 )4.72 × 10−12
2.58 × 10−12
1.19 × 10−12
6.81 × 10−13
9.20 × 10−14
2.32 × 10−14

TABLE IIO UTER G ALACTIC PLANE
Fig. 2.
Upper limit of the ratio of isotropic gamma rays tocosmic rays. The results of the HEGRA-AIROBICC [11], UtahMichigan [12], EAS-TOP [13] and CASA-MIA [14] experiments andprevious (1999 [8], 2001 [9]) and this work with the GRAPES-3
experiment are shown by the points, as indicated in the legend.

>E(TeV)
17.6
27.9
44.3
70.2
111
176

Significance
(σ)
−2.34
0.56
−2.63
0.20
0.58
−1.22

90%C.L. upper limit(cm−2 s−1 sr−1 )4.73 × 10−12
3.41 × 10−12
8.42 × 10−133.37 × 10−13
1.42 × 10−13
2.25 × 10−14

Fig. 4. The experimental results for diffuse gamma rays from theinner galactic plane. The upper limits from the Tibet AS-γ [15] andWhipple [16] experiments are shown by the bars with arrows. The
definite flux from Milagro experiments [17] is shown by the bar witherror.

Fig. 3. Distribution of the number of events in the galactic latitude
range of −20◦ to 20◦ . The background level at the galactic plane region is determined from fitting the number of events at the backgroundregion ignoring the region of the galactic latitude 2◦ < |b| < 4◦ .

90% confidence level upper limit at different energies
are listed in Table I and II.
Fig. 4 and 5 show the flux upper limits for the innergalactic plane and for the outer galactic plane.
VI. C ONCLUSIONS
We obtained the upper limits of diffuse gamma rays
based on the observations from March 2000 to JulyFig. 5. The experimental results for diffuse gamma rays from theouter galactic plane. The upper limits from the Tibet AS-γ [15] andMilagro [17] experiments are shown by the bars with arrows.

2004 with the GRAPES-3 air shower array. A search
for isotropic diffuse gamma rays resulted in a 90% C.L.
upper limit of 5.5 × 10−5 at 280 TeV. And we also
obtained the upper limits of gamma ray flux from theinner and outer galactic planes.

4M. MINAMINO et al. GRAPES-3 DIFFUSE EMISSIONACKNOWLEDGEMENTS
The GRAPES-3 experiment is supported by theMinistry of Education, Culture, Sports, Science andTechnology-Japan through Kakenhi grants on basic research 15403005(B). We also wish to thank the valuable contributions of G. Paul Francis, V. Jeyakumar,
K. Manjunath, K. Ramadas, B.S. Rao, C. Ravindranand V. Viswanathan during the installation, operationand maintenance of the instrumentation. We gratefully
acknowledge the help and cooperation of the Radio Astronomy Centre for providing facilities for the GRAPES3 air shower array.
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